Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse... – Magnetic field and light beam
Reexamination Certificate
1999-05-03
2004-02-03
Dinh, Tan (Department: 2653)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
Magnetic field and light beam
C369S044230, C369S118000, C369S044240, C369S112240
Reexamination Certificate
active
06687196
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical head and disk apparatus which use near field wave, and a method for manufacturing optical heads, and more particularly, relates to an optical head which implements high density recording on a recording medium and a small-sized optical head of improved data transfer rate, a disk apparatus, a method for manufacturing optical heads, and an optical element for an optical head.
2. Description of Related Art
In the field of optical disk apparatus, the optical disk has changed historically from the compact disk (CD) to the digital video disk (DVD), which has a large recording capacity and is capable of high density recording. The recent development of high performance computers and high resolution displays has resulted in increasing demand for large capacity recording.
The recording density of an optical disk depends basically on the diameter of an optical spot formed on a recording medium. Recently, the near field wave technology in the field of the microscope has been applied to the optical recording technology as a technology for miniaturizing the beam spot diameter. As the conventional optical disk apparatus which uses the near field wave, for example, the optical disk described in the literature (Jpn. J. Appl. Phys., Vol. 35 (1996) P. 443) and U.S. Pat. No. 5,497,359 has been known.
FIG.
23
(
a
) and FIG.
21
(
b
) show an optical disk apparatus described in the literature (Jpn. J. Appl. Phys., Vol. 35 (1996) P. 443). As shown in FIG.
23
(
a
), the optical disk apparatus
190
is provided with a semiconductor laser
191
that emits a laser beam
191
a
, a coupling lens
192
that changes the laser beam
191
a
emitted from the semiconductor laser
191
to a collimated beam
191
b
, and an optical fiber
193
which is polished in a taper shape having a larger diameter at the incident end
193
a
and a smaller diameter at the emission end
193
b
, and provided with a probe
194
that introduces the collimated beam
191
b
which comes from the coupling lens
192
from the incident end
193
a
, and a recording medium
195
on which the information is recorded by means of the near field wave
191
c
that leaks from the emission end
193
b
of the optical fiber
193
.
The recording medium
195
has a recording layer
195
a
consisting of GeSbTe, which is a phase change recording medium, which recording medium is heated by incident near field wave
191
c
, and then the heating causes phase change between crystal/amorphous, and difference in reflectance between both phases is utilized for recording.
The optical fiber
193
has the incident end
193
a
having a diameter of 10 &mgr;m and the emission end
193
b
having a diameter of 50 nm, and is coated with a metal film
194
b
consisting of a metal such as aluminum with interposition of a clad
194
a
to prevent the beam from leaking to somewhere other than the emission end
193
b
. The diameter of the near field wave
191
c
has the approximately same diameter as the diameter of the emission end
193
b
, therefore the high density recording of several 10 Gbits/inch
2
is possible.
For reproduction, as shown in FIG.
23
(
b
), a near field wave
191
c
having such a low power as it does not cause phase change is irradiated onto the recording layer
195
a
by use of the same optical head as used for recording, the reflected beam
191
d
from the recording layer
195
a
is condensed on a photomultiplier
197
by means of a condenser lens.
FIG. 24
shows an optical head of an optical disk apparatus disclosed in U.S. Pat. No. 5,497,359. The optical head
50
is provided with an condense lens
52
that condenses a collimated beam
51
and an Super SIL (Super Solid Immersion Lens)
54
having the form of bottom-cut sphere placed with the bottom plane
54
a
perpendicular to the condensed beam
53
from the condense lens
52
. The collimated beam
51
is condensed by the condense lens
52
and the condensed beam
53
is incident onto the spherical incident surface
54
b
, the condensed beam
53
is refracted at the incident surface
54
b
and condensed on the bottom surface
54
a
to form a beam spot
55
on the bottom surface
54
a
. Because the wavelength of the beam becomes short in inversely proportional to the refractive index in the internal of the super SIL
54
, the diameter of the beam spot becomes small in proportion to it. A part of the beam condensed on the beam spot
55
is totally reflected toward the incident surface
54
b
, but the beam leaks partially from the beam spot
55
to the outside of the super SIL
54
as a near field wave
57
. A recording medium having the approximately same refractive index as that of the super SIL
54
is located at the close distance from the bottom surface
54
a
so that the distance is sufficiently smaller than a wavelength of the wave, then the near field wave
57
is coupled with the recording medium
56
and propagates in the recording medium
56
. The information is recorded on the recording medium
56
by the propagation beam.
By structuring the Super SIL
54
so that the collimated beam
51
is condensed at the position r
(r denotes the radius of the Super SIL) distant from the center
54
c
of the semi-spherical surface
54
b
, the spherical aberration due to the Super SIL
54
is reduced and the numerical aperture in the Super SIL
54
is increased, and further the diameter of the beam spot
55
is minimized. In detail, the beam spot
55
is minimized according to the equation 1.
D
½
=k
&lgr;/(
n·NAi
)=
k
&lgr;/(
n
2
·NAo
) (1)
where,
D
½
: beam spot diameter where the intensity becomes a half of the maximum intensity.
k: proportional constant (normally around 0.5) which depends on the intensity distribution of an optical beam
&lgr;: wavelength of an optical beam
n: refractive index of an Super SIL
54
NAi: numerical aperture in an Super SIL
54
NAo: numerical aperture of an incident beam to an Super SIL
54
The collimated beam
51
is condensed as the beam spot
55
without absorption on the optical path and high optical utilization factor is obtained. As the result, a beam source having a relatively low output is sufficient for use and the reflected beam is detected without a photomultiplier.
PROBLEM OF THE RELATED ART
However, according to the conventional optical disk apparatus
190
, though a beam spot having a size of several ten nm is formed on a recording medium, a laser beam which enters an optical fiber
193
is partially absorbed in its inside due to the taper shape of the optical fiber
193
, and the optical utilization factor is as low as {fraction (1/1000)} or lower disadvantageously. Because of the low optical utilization factor, a photomultiplier
197
is undesirably required to detect the reflected beam
191
d
and the photomultiplier leads to a large sized as well as expensive optical head. Further, slow response speed of the photomultiplier
197
and heavy weight optical head result in the slowed-down tracking speed. Due to many problems such as a low transfer rate due to slow rotation of an optical disk, much improvement is required for practical use.
FIG. 25
is a graph for describing the problem of the conventional optical head
50
shown in
FIG. 24
, which was presented by T. Suzuki in #OC-1 in Asia-Pacific Data Storage Conference (Taiwan, 1997. 7), the relation between the refractive index n of a SIL
54
and NAo is shown. There is a reversal relation between NA of incident beam to the SIL
54
, namely the maximum value &thgr;max of the incident angle &thgr;, and the refractive index n of the SIL
54
, and the two values cannot be increased independently. It is understandable as shown in the graph that the possible maximum value NAomax of NAo of the incident beam becomes gradually smaller with increasing of the refractive index of the SIL
54
, because the beam having a large incident angle which is caused from increased NAo larger than the maximum NAomax enters directly into the recording medium
56
without passing through
Chu Kim-Kwok
Fuji Xerox Co., Lt.d.
Oliff & Berridg,e PLC
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